Pyrolysis is a thermal process for breaking down hydrocarbon materials in the absence of oxygen into smaller constituent materials, including carbon and hydrocarbon compounds with a wide range of molecular weights and gases. When feedstock consists of organic polymers, pyrolysis causes the polymer backbone to decompose and the products can include carbon char and condensable and non-condensable gases.
During tire pyrolysis, chemical bonds within the rubber compounds are broken down, creating a mixture of organic compounds and non-condensable gases. Carbon black, present as a major component in the tire polymer matrix, is freed. Other inorganic compounds, such as silicon dioxide, zinc oxide and aluminum oxide, present within the tire are also freed from the polymer matrix. Organic compounds within the matrix, consisting of larger carbon chains (C6 and larger), sublime to a gaseous state at normal operating temperatures and include a mixture of aromatic, aliphatic and olefinic hydrocarbons. Non-condensable gases such as methane, ethane, propane, hydrogen, carbon monoxide and hydrogen sulfide are also formed during pyrolysis. Additional carbon black, in minor amounts, is also formed when carbon is split off from the polymeric chains and is carbonized. The end products of a tire pyrolysis process typically include carbon black, pyrolysis oil, non-condensable gases, and inorganic ash.
Several problems are believed to hinder the technical and commercial viability of commercial pyrolysis systems. For example, the pyrolysis process and the resulting products are highly dependent on a number of variables including the type, size, and shape of feed material; pyrolysis conditions such as the pyrolysis rate, the processor type, thermal and gas flow gradients within the processor; gas and carbon co-mixing and exiting the processor; and methods for effectively recovering and separating desired products.
Further, the pyrolysis gas stream exiting from the pyrolysis system typically contains a mixture of condensable and non-condensable gases and a small portion of fluidized carbon black and inorganic ash particles, which become entrained in the gas stream. Certain chemicals in the gas, particularly the olefins and aromatics, have an affinity for the carbon particles and begin to condense on the carbon surface. Other chemicals in the gas, particularly polar compounds, will condense on the inorganic ash particles. This leads to carbon and inorganic ash mixtures that will adhere on surfaces, causing undesired buildup throughout the system. This can eventually lead to excessive fouling and plugging. Ultimately, this can lead to added maintenance and downtime for cleaning. If the condensable pyrolysis-gases are to be collected by methods such as condensation, the pyrolysis oil or pyrolysis gas contaminated with carbon black and inorganic ash can be an unacceptable contaminant, degrading product purity. If the light, non-condensable gases are to be collected or burned, the entrained carbon and inorganic ash will ultimately foul tubing, valves, pumps, compressors, burners or other equipment.
The following provide examples of pyrolysis systems: U.S. Pat. No. 7,329,329 issued Feb. 12, 2008; U.S. Pat. No. 6,736,940 issued May 18, 2004; U.S. Pat. No. 6,221,329 issued Apr. 24, 2001; U.S. Pat. No. 6,149,881 issued Nov. 21, 2000; U.S. Pat. No. 6,048,374 to Green; U.S. Pat. No. 5,225,044 issued Jul. 6, 1993; U.S. Pat. No. 5,037,628 issued Aug. 6, 1991. Each of the foregoing patents is incorporated by reference in its entirety into this application.
The following provide examples of further post-processing and uses for the resulting products of pyrolysis: U.S. Pat. No. 7,416,641 issued Aug. 26, 2008; U.S. Pat. No. 7,101,463 issued Sep. 5, 2006; U.S. Pat. No. 6,322,613 issued Nov. 27, 2001; U.S. Pat. No. 6,103,205 issued Aug. 15, 2000; U.S. Pat. No. 5,894,012 issued Apr. 13, 1999; U.S. Pat. No. 4,839,151 issued Jun. 13, 1989. Each of the foregoing patents is incorporated by reference in its entirety into this application.
The formation of zinc sulfide during pyrolysis of tires is also reported in the following literature, which is incorporated by reference in its entirety into this application: The Vacuum Pyrolysis of used tires. End use for the oil and carbon black products; C Roy, A Chaala, and H. Darmstadt; Journal of Analytical and Applied Pyrolysis, 51 (1999) 201-221.